The invention relates to a differential drive having a drivable differential carrier supported in a drive housing so as to be rotatable around an axis, having two axle shaft gears which are coaxially supported one behind the other in said differential carrier in first bores and which are each provided with outer toothing and which are connectable to axle shafts emerging from the differential carrier, having a plurality of pairs of differential gears which are supported eccentrically and in an axis-parallel way in said differential carrier in second and third bores and which are each provided with outer toothing covering a portion of the length of same and with hub extensions of different lengths adjoining said outer toothing at both ends, with the differential gears of the individual pairs being oriented in different directions relative to one another, with the first bores intersecting the second and third bores and the latter ones intersecting one another, and with the first axle shaft gear engaging the first differential gears of the individual pairs and with the second axle shaft gear engaging the second differential gears of the individual pairs and with the differential gears of the individual pairs engaging one another in at least one axially overlapping region of their outer toothing and with the differential gears, for the purpose of generating friction forces, being supported in the second and third bores by means of the tooth heads of their outer toothing.
Differential drives of this type are known from DE 37 07 872 A1 for example. In the case of differential movements between the axle shaft gears in the differential drive, the differential gears are supported in the second and third bores by means of their tooth heads, thus generating considerable friction forces which adversely affect the efficiency of the differential drive. This specifically causes a slip-limiting effect between the axle shafts. The differential gears of the prior art drive comprise so-called hub extensions by means of which they are axially supported on end faces of their respective bores. Said hub extensions ensure that the differential gears are accurately axially positioned in the bores for the differential gears, which bores are uniformly extending over the entire housing length. Such support with the help of hub extensions is necessary, especially in the case of the helical gearing of the axle shaft gears and differential gears of prior art drives, which helical gearing generates axial forces. The hub extensions do not have a bearing function.
The direction and magnitude of the above-mentioned supporting forces of the differential gears relative to the differential carrier in the case of differential movements in the differential carrier depend on the geometric relations such as diameter ratios and angles of helical pitch as well as torque introduced. As a result of the mutual engagement of teeth, forces comprising a tangential component may be introduced into the differential carrier. Whereas normally, the resulting radial component of such forces is directed outwardly, it is possible for an inwardly directed radial component to occur as well.
Therefore, in the case of differential drives of said type, the edge regions along the lines of intersection of said second and third bores relative to one another, as well as the edge regions along the lines of intersection of said second and third bores relative to the first bores, are subject to particularly high loads. This may lead to excessive wear and edge fracture, which represent undesirable results and which may affect the predetermined slip-limiting effect of the drive.
From U.S. Pat. No. 5,122,101 there is known a differential drive of a similar design wherein the second and third bores in the differential carrier are also of the same length and wherein the differential gears comprise corresponding axial extensions. However, said axial extensions do not serve as axial spacing members, as described above, but as radial bearing journals by means of which the differential gears, at both ends, are supported in end housing parts. In consequence, the differential gears are held with play in their bores so that the tooth heads run in a friction-free way relative to the bores. As far as, with a differential drive of this type, there occurs a slip-limiting effect between the axle shafts, it occurs entirely as a result of the axial thrust which is generated by helical toothing of the axle shaft gears and differential gears and leads to friction forces of the axle shaft gears and differential gears at the end faces of the differential carrier. The slip-limiting effect is thus correspondingly lower.
It is the object of the present invention to provide a drive of the initially mentioned type, i.e. a drive wherein friction is generated at the tooth heads of the differential gears relative to the differential carrier and wherein there are provided means which relieve the load on said surface regions of the differential carrier in the region near the edges and which are formed by the second and third bores penetrating one another and each penetrating the first bores and which permit a precision adjustment of the slip-limiting effect.
The objective is achieved in that at each differential gear, the longer hub extension distal from the region of engagement with the associated axle shaft gear is fixed radially and that the shorter hub extension adjoining the region of engagement with the associated axle shaft gear is laterally guided in the radial direction. This measure ensures that the circumferential components of the forces resulting from the tooth forces between the differential gears are introduced into the differential carrier by the hub extensions whereas the outwardly directed radial components of the resulting forces applied to the differential gears are introduced in a friction-generating way by means of the tooth heads into the surface parts of the second and third bores in regions which extend substantially tangentially relative to the housing axis. Said circumferential components of the resulting forces subjecting the edges to loads are thus eliminated, whereas the function of the friction forces increasing slip-limiting effect is maintained in principle. The dangerous edge pressure is avoided in an advantageous way.
To avoid any indifferent conditions of engagement between the pairs of differential gears, the longer hub extension distal from the region of engagement with the associated axle shaft gear is fixed radially, whereas, in accordance with the invention, the shorter hub extension adjoining the region of engagement with the associated axle shaft gear is laterally guided with radial play to be able to generate the friction forces in the bores, which friction forces are essential for this type of differential drive.
According to the preferred embodiment it is proposed that at each differential gear, the shorter hub extension adjoining the region of engagement with the associated axle shaft gear is supported radially inwardly. This measure allows any possible inwardly directed radial components of the resulting forces applied to the differential gears to be introduced by the hub extensions into the differential carrier in such a way that especially those forces are eliminated which act on the edges in the region where the first and second bores penetrate the third bore.
In an advantageous embodiment, there are provided pairs of bearing plates for jointly supporting and guiding adjoining hub extensions of the pairs of differential gears, which bearing plates each comprise a centric bearing bore for the longer hub extension and an oblong hole--extending radially relative to the drive axis--for the shorter hub extension of each of the two adjoining differential gears of a pair of differential gears. In respect to their bores or oblong holes, said pairs of bearing plates are arranged at the two ends of the housing so as to be orientated in opposite senses. Said bearing plate embodiment which optimizes the engagement of the toothings of the two differential gears of one pair may comprise an angle of the longitudinal axis of the oblong hole, which deviates from the entirely radial direction in order to influence the friction forces at the tooth heads in the second and third bores. Using said bearing plates is also advantageous in that, with an unchanged housing geometry, it is possible to use different sets of gears in a differential drive. In this way it is possible to modify certain design aspects without having to introduce an entirely new layout of the overall concept. It is possible to vary the rolling circle of the axle shaft gears and the rolling circle of the differential gears as well as the helical pitch of the toothing, with the differential carrier being adapted by exchanging the bearing plates.
According to an advantageous embodiment, the bearing plates--if viewed in a cross-section through the drive--are form-fittingly inserted into the differential carrier. Suitable recesses for the bearing plates may be provided in cover parts or in an axial central portion of the differential carrier.
Normally, the differential carrier consists of an axial central portion provided with the first, second and third bores, and two covers which close the bores.
As known in itself, and as especially described in the state of the art quoted, the bores may all be through-bores in the central portion, with, normally, an axial spacing piece being inserted into the first bore between the two axle shaft gears. In a preferred embodiment, the pairs of differential gears engage one another only in the central axial portion between the regions of toothing of the axle shaft gears.
If said bearing plates are not used, the bearing bores and oblong holes in accordance with the invention should preferably be provided in the housing covers. To modify the layout, the differential carrier may be adapted by changing the cover plates while using an unchanged central portion.
A preferred embodiment will be described below with reference to the drawings.